The emerging field of lightwave telecommunications makes use of light to transmit information through a transparent medium in a similar way to electricity through copper or aluminum wire. Since the information-carrying capacity of electromagnetic radiation increases with frequency, the communications traffic that could be accommodated in the bandwidth of visible radiation (light) is potentially many thousands of times that of radio communications.
Since the discovery of a suitable light source in the laser around 1960, the only technical obstacle to lightwave communications over great distances was the development of a suitable transmission medium. Air, for example, although penetrable by light, was unsuitable because rain, fog, and other atmospheric conditions could weaken (or "attenuate") the light signal. Development of the glass fiber lightguide, or optical fiber, provided an excellent and relatively inexpensive transmission medium.
Modern optical fibers typically consist of a core of high transparency silica glass, which transmits the light, surrounded by a transparent coating of lower refractive index than the core. The coating acts as an internal mirror, reflecting the light back into the core and thus preventing loss of the light signal outside the optical path.
While the lower refractive index coating theory has provided serviceable lightguides for relatively short-distance telecommunications (e.g., building-to-building or intramural), for long-distance telecommunications (e.g., transcontinental), where many lightguides may be bundled together, the problem of signal "noise" becomes more important than signal attenuation. To a particular information-carrying lightwave, any incidental lightwaves (carrying other information) or signals disrupting to the first lightwave are "noise" from which the desired information must be extracted. It has been found that signal noise can be minimized in lightwave transmissions by coatings of higher refractive index than the core fiber.
There is thus a continuing search for coating materials having either a higher or a lower refractive index than the core fiber material For silica glass optical fibers, the reference point is 1.47, the refractive index of silica glass fiber. Suitable materials will have refractive indices lower, preferably less than 1.45, or higher, preferably greater than 1.50.
In the production of fiber optics cable for telecommunications, the material used for primary coatings must be very flexible, must not adhere too closely to the glass fiber core (to permit joining and other manipulations), and must maintain its integrity and optical characteristics in changeable environments, including temperature cycles of from -60.degree. to +80.degree. C.
Many fiber optics producers have adopted heatcurable polydimethylsiloxane coating compositions as the primary lightguide coating. The uncoated optical fiber is typically drawn through the silicone composition, then through an eight-inch oven at 800.degree. C. for curing. The time required to fully cure the silicone composition has become the limiting factor in increasing line speeds in producing optical fibers: Since higher oven temperatures (or longer ovens) cause oxidation of the silicone and also begin to affect the drawn fibers, line speeds cannot be increased beyond about 30 meters/minute with commercially available thermally cured silicone coatings.
The desire to attain higher production speeds has led optical fiber producers to investigate ultraviolet radiation (UV)-curable materials, but a coating composition having a combination of properties comparable to the silicone materials has not as yet been found.
It has now been discovered that certain UV-curable polysiloxane compositions provide novel coating materials for optical fibers which exhibit the desired combination of properties for optical fiber cladding layers. The discovery includes both low refractive index and high refractive index compositions, all of which cure rapidly on brief exposure to ultraviolet radiation, thus offering significant advantages in safety, cure rate, and cost over thermally cured silicone materials.